Semiconductor devices are constructed of patterned layers of electrically conductive, non-conductive and semi-conductive materials stacked over a silicon wafer. The layers of material are successively deposited on the wafer and etched into predefined patterns to form individual component structures within the particular device being fabricated. The manufacturing process typically also includes the heat treatment of the silicon wafers at different stages of manufacture in a thermal processing furnace, sometimes referred to as a "diffusion" furnace. Diffusion furnaces are used for heat treatment processes such as impurity diffusion, oxide growth, chemical vapor deposition ("CVD"), annealing and the like. The furnaces basically include a furnace tube surrounded by a heating element. The wafers are placed in the furnace tube in specially made cassettes or "boats" wherein the wafers are arranged parallel to one another and oriented so that the surface of each wafer is perpendicular to the longitudinal axis of the furnace tube. The diameter of the furnace tube is somewhat greater than the diameter of the wafers.
During processing, the wafers are heated to the desired temperature and inert and reactant gases are introduced into the furnace tube. Gases introduced into one end of the furnace tube thus flow over the wafers axially along the length of the furnace tube and are exhausted at the other end. A region of laminar flow exists between transition areas formed by velocity gradients in the gases flowing axially along the inner surface of the tube and along the edges of the wafers. A region of turbulent flow exists fully across the surfaces of the wafers. This turbulent region and the corresponding pressure drop promotes a more uniform reaction between the gases and the surface materials of the wafers. The present invention is directed to an improved furnace tube wherein the transition area formed along the inner surface of the tube is extended outward to narrow the laminar region of the flow of gases through the tube. Narrowing the laminar region of flow causes higher turbulence in the turbulent region, which increases the pressure drop across the surface of the wafers, and results in a more uniform reaction across the surface of the wafers.